Spread spectrum communication is a type of signal modulation in which a signal to be transmitted is spread over a bandwidth that substantially exceeds the data transfer rate or the minimum bandwidth required to transmit the signal. Fundamentally, in channels with narrowband noise, increasing the transmitted signal bandwidth decreases the interference effect of such noise since the signal instead of being concentrated in a particular band is now spread out over a much wider band.
Signal spreading may be achieved using any of a number of different techniques, including direct sequence, frequency hopping or a hybrid combination. In direct sequence spread spectrum, a periodic, relatively high frequency, repetitive pseudo-noise code (PN) is mixed with the data signal using XOR gates or a mixer, and the resulting signal is then modulated using, e.g., binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK). This process causes the transmitted signal to be replaced by a very wide bandwidth signal with the spectral equivalent of a noise signal. At the receiver, the demodulation process involves the mixing and multiplying of the same PN code with the received signal. This produces a correlated signal which is maximum when the PN code matches the received signal. The correlated signal is then filtered and demodulated.
In frequency hopping spread spectrum, the signal stream to be transmitted is shifted in frequency by an amount determined by a code that spreads the signal power over a wide bandwidth. This is accomplished using a PN code controlled frequency synthesizer. In this way, the instantaneous frequency output of the transmitter jumps from one value to another based on the PN code. By changing the instantaneous frequency of the output signal, the output frequency spectrum is effectively spread over the range of the wider bandwidth.
The PN codes used in direct sequence spread spectrum systems consist of individual units called “chips” which can have two values, either −1/1 in a polar system or 0/1 in a binary system. In general, these PN codes must have a sharp (e.g., one chip wide) autocorrelation peak to allow for proper code synchronization, as well as a low cross-correlation value to allow for more users in the system. Additionally, the codes should be balanced, i.e., the number of ones and zeros may only differ by a maximum of one. This latter requirement allows for good spectral density such that the signal energy may be uniformly spread over the entire frequency band. Codes which may be used in direct sequence spread spectrum systems include Walsh-Hadamard codes, M-sequences, Gold codes and Kasami codes. These codes may be either orthogonal or non-orthogonal.
One difficulty encountered in spread spectrum systems is achieving proper synchronization, i.e., having the receiver lock onto and synchronize with the bit timing of the transmitter so that the receiver is sampling and segmenting the received signal at the proper bit boundaries intended by the transmitter. One approach to achieving synchronization is disclosed in U.S. Pat. No. 5,727,004 entitled METHOD AND APPARATUS FOR DATA ENCODING AND COMMUNICATION OVER NOISY MEDIA, and assigned to the assignee of the present application, the contents of which are incorporated herein by reference. The synchronization procedure disclosed in U.S. Pat. No. 5,727,004 eoversamples the received signal at a number of points which are offset in time by a fraction of the bit interval (i.e., sub-interval). Each of the sampled signals is analyzed to determine which of the bit intervals results in good correlation and therefore represents the proper bit boundaries. The oversampling of the incoming signal at a number of sub-intervals generally requires either a clock or some other timing source running at a multiple of the bit clock in order to sample the signal at subinterval increments.
The FCC regulations governing electromagnetic radiation limits, 47 CFR § 15.107, 109, 209, . . . place limits on radiated and conducted emissions for both intended and unintended emissions. The term “intended” refers to the intended mode of transmission. For example, in the case of wireless communication, the “intended” mode of transmission is radiated, while the unintended mode of transmission is conducted. Similarly, in the case of wired communication, the “intended” mode of transmission is conducted, while the unintended mode of transmission is radiated.